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From: jer@peora.UUCP (J. Eric Roskos)
Newsgroups: net.mail
Subject: Re: Neighbor optimized away
Message-ID: <2124@peora.UUCP>
Date: Thu, 1-May-86 12:08:08 EDT
Article-I.D.: peora.2124
Posted: Thu May  1 12:08:08 1986
Date-Received: Sat, 3-May-86 18:59:58 EDT
References: <4509@dartvax.UUCP>
Organization: Concurrent Computer Corporation, Orlando, Fl
Lines: 32
Keywords: uucp pathalias

> It turns out that the pathalias "cost" of our link to Z is greater than
> that of path A!B!C!Z, so the optimizer always chooses A!B!C!Z.  This
> effectively makes our link to Z useless, which somehow doesn't seem right.

Alas, this is really just a case where what "seems right" conflicts with
what you've more formally defined to *be* right!  if the cost of the link
to Z is greater than the path of A!B!C!Z, then why wouldn't pathalias (which
is really what's doing it) choose the "longer" path?  Its optimization is
based on the costs, not the number of sites.

I find that people around here also comment on that occasionally... "how
come the router generated a path with five sites in it when I know of
one that has only three?"  Because pathalias is trying to get the mail there
while minimizing the "cost", where the cost is supposed to be (if you use
the constants like LOCAL, DAILY, WEEKLY, etc) a function of how fast it
gets there.  If you tell pathalias that it's faster to send it the "long"
way, then it will send it that way... the only inconsistency is in what
intuitively seems to be the "right" path!  If, in fact, it would have gotten
there faster if you'd gone via the 1-hop path, then you should adjust the
weights in the map.  (Note that if it's high because your management or
administration has a policy of trying to minimize outside traffic through
the link -- which would certainly be reasonable if you had some convenient
but expensive connection that was funded for some specific purpose other
than the general mail -- you could publish a different weight in the
published maps than what you use when you generate your pathalias database.)

[I know someone will say "the `costs' are not that simple," since
the semantics of the cost are sort of arbitrarily assigned -- for example
some people may assign higher costs because it is more expensive to use
a given link -- I realize that, the above is something of a simplification.]
-- 
E. Roskos